Method for enhancing reactivity of pozzolanic materials

ABSTRACT

Preparing a cement slurry by mixing at least water and a cement dry blend, wherein the cement dry blend comprises a cement and an activated pozzolan; and introducing the cement slurry into a wellbore penetrating a subterranean formation; and allowing the cement slurry to set to form a hardened mass.

BACKGROUND

In well cementing, such as well construction and remedial cementing,cement slurries are commonly utilized. Cement slurries may be used in avariety of subterranean applications. For example, in subterranean wellconstruction, a pipe string (e.g., casing, liners, expandable tubulars,etc.) may be run into a well bore and cemented in place. The process ofcementing the pipe string in place is commonly referred to as “primarycementing.” In a typical primary cementing method, a cement slurry maybe pumped into an annulus between the walls of the well bore and theexterior surface of the pipe string disposed therein. The cement slurrymay set in the annular space, thereby forming an annular sheath ofhardened, substantially impermeable cement (i.e., a cement sheath) thatmay support and position the pipe string in the well bore and may bondthe exterior surface of the pipe string to the subterranean formation.Among other things, the cement sheath surrounding the pipe stringfunctions to prevent the migration of fluids in the annulus, as well asprotecting the pipe string from corrosion. Cement slurries also may beused in remedial cementing methods, for example, to seal cracks or holesin pipe strings or cement sheaths, to seal highly permeable formationzones or fractures, to place a cement plug, and the like.

A particular challenge in wellbore cementing may be to ensure thatcements have consistent and predictable properties. Pozzolans are usedin cement slurry designs for multiple purposes such as yieldenhancement, CO₂ footprint reduction, and increased mechanicalproperties, for example. However, pozzolans vary greatly in theirreactivity with some being highly reactive and others having very lowreactivity, and some may be inert in typical oilwell conditions. Thevariation in pozzolan reactivity may lead to variation in cement designsdepending on what pozzolans are available. Field camp locations withhighly reactive pozzolan sources can design slurries which have a muchhigher proportion of pozzolan, while those with low reactivity pozzolansare limited to low concentrations since these pozzolans do notcontribute as much to strength development or other properties. Cementslurries prepared with pozzolans with relatively lower reactivity maynot have the desired properties for oilwell cementing.

In the present disclosure, an activated pozzolan additive is disclosedwhich may be included in cement slurries for oilwell cementing. Thepresent methods may utilize the activated pozzolan additive in the bulkmixing preparation of a dry cement blend. The dry cement blend may betransported to a well site and mixed with water to produce a cementslurry which may be introduced into a wellbore.

BRIEF DESCRIPTION OF THE DRAWINGS

These drawings illustrate certain aspects of some of the embodiments ofthe present disclosure and should not be used to limit or define thedisclosure.

FIG. 1 is a schematic illustration of an example system for thepreparation and delivery of a cement slurry to a wellbore.

FIG. 2 is a schematic illustration of example surface equipment that maybe used in the placement of a cement slurry in a wellbore.

FIG. 3 is a schematic illustration of the example placement of a cementslurry into a wellbore annulus.

FIG. 4 is a graph of a crush corrected UCA of raw recycled glass versusactivated pozzolan.

FIG. 5 is a graph of a crush corrected UCA of raw perlite versusactivated pozzolan.

DETAILED DESCRIPTION

The present disclosure may generally relate to cementing methods andsystems. More particularly, embodiments may be directed methods ofpreparing oilwell cement slurries from a dry cement blend whichcomprises an activated pozzolan.

As used herein, activated pozzolan refers to a raw pozzolanic materialwhich has been surface modified by reacting the raw pozzolanic materialin a passivated cement solution. The passivated cement solution mayimpart several properties to the surface of the raw pozzolanic materialto form the activated pozzolan, including without limitation, surfaceetching, formation of microcrystalline calcium silica hydrate on thesurface, dissolution of silicates to form more reactive silicatespecies, and others. The activated pozzolan has greater reactivity thanthe raw pozzolanic material the activated pozzolan is derived fromthereby allowing the activated pozzolan to be utilized in a widervariety of cement designs than the corresponding raw pozzolanicmaterial. The process described herein may be utilized to upgraderelatively lower reactivity pozzolans to relatively higher reactivitypozzolans thereby increasing the value of the lower reactivity pozzolan.Activated pozzolan may be included in a bulk dry cement blend. Cementdry blends may be prepared in bulk blending facilities where a cement,such as Portland cement, may be mixed with additives such assupplementary cementitious materials, chemical additives, inertadditives, and the activated pozzolan. While there are many advantagesto including the activated pozzolan in a wellbore cement slurry, aparticular advantage may be reduction in greenhouse gas emissions as aportion of the cement may be replaced by activated pozzolan. Anotheradvantage is that the mechanical properties of a set cement, such ascompressive strength and Young's modulus, are increased by including theactivated pozzolan as compared to a cement containing the raw pozzolanicmaterial.

Pozzolans are typically classified as materials containing siliceousand/or aluminous materials which react with water and calcium hydroxideto form a set cement. Any suitable raw pozzolan may be utilized in thepresent application to produce activated pozzolan, including forexample, fly ash, volcanic ash, tuft, pumicites, clays such asmetakaolin, silica fume, slag, lime ash, perlite, and glass such assilicate glass, soda-lime glass, soda-silica glass, borosilicate glass,aluminosilicate glass, aplite, clays, and calcined clays. While thepresent list is of suitable raw pozzolans is non-exhaustive, it isbelieved that any pozzolan suitable for use in an oilwell cement is alsosuitable for the processes described herein to produce activatedpozzolan. The raw pozzolan may have any particle size distribution andmorphology. In certain embodiments, the pumice may have a d50 particlesize distribution in a range of from about 1 micron to about 200microns, or greater. The d50 values may be measured by particle sizeanalyzers such as those manufactured by Malvern Instruments,Worcestershire, United Kingdom. In specific embodiments, the rawpozzolan may have a d50 particle size distribution in a range of fromabout 1 micron to about 200 microns, from about 5 microns to about 100microns, or from about 10 microns to about 25 microns.

Preparing the activated pozzolan comprises preparing a passivated cementsolution A passivated cement solution is a solution which containscement and water in an amount such that the passivated cement solutiondoes not set to form a hardened mass at 20° C. and 101.325 kPa whentested in a ultrasonic cement analyzer, operated in accordance with theprocedure set forth in the aforementioned API RP Practice 10B-2. Inwellbore cementing, water is added to a dry cement in an amount toproduce a cement slurry with a desired density and in an amount suchthat the cement slurry will set to from a hardened mass with a desiredcompressive strength. In wellbore cementing, density is an importantproperty when the cement is being pumped into the wellbore to ensurethat sufficient pressure is exerted by the cement slurry and the wellremains controlled. When the cement slurry sets, the compressivestrength is an important property to ensure zonal isolation does notbecome compromised. Thus, there is a minimum amount of water to includewhich begins to hydrate cement grains and maximum practical amount ofwater that would be added to a dry cement blend when preparing a cementslurry for wellbore use to meet density and compressive strengthrequirement, among others. As more water is added to a cement slurrybeyond what is required to hydrate the individual grains of cement, thecement grains begin to disperse in solution. The dispersive effectslower the inter-grain formation of cement hydration products therebyreducing the compressive strength of the set cement until there isenough water to separate the grains such that the solution does not setto form a hardened mass.

The passivated cement solutions of the present application are distinctfrom cement slurries prepared for use in wellbore or constructionapplications as the passivated cement solution contains water in anamount which prevents the passivated cement solution from setting toform a hardened mass. Cement slurries prepared for wellbore andconstruction cementing contain water in amounts that allow the cementslurry to set to form a hardened mass. In some examples, passivatedcement solution may have a density close to that of water (1 kg/l). Forexample, the passivated cement solution may have a density from about 1kg/l to about 1.2 kg/l. However, density of the passivated cementsolution is dependent upon the amount of water required to form asolution which does not set to form a hardened mass. The amount of waterrequired to form a solution that does not set may vary greatly betweencements. While low density cements slurries do exist which have lowerdensity than water, these cements slurries are usually formulated withlight weight beads or a foaming surfactant and foaming gas which reducesthe density below that of water. However, low density cements slurriesstill contain water in amounts that allow the cement to set to form ahardened mass and are distinct from the passivated cement solution whichdoes not set.

Thickening time typically refers to the time a fluid, such as a cementcomposition, remains in a fluid state capable of being pumped. A numberof different laboratory techniques may be used to measure thickeningtime. A pressurized consistometer, operated in accordance with theprocedure set forth in the aforementioned API RP Practice 10B-2, may beused to measure whether a fluid is in a pumpable fluid state. Thethickening time may be the time for the treatment fluid to reach 70 Bcand may be reported as the time to reach 70 Bc. In wellbore cementing,70 Bc (Beardan units of consistency) is used as a cutoff for when acement is considered too set to pump. The passivated cement solution asdisclosed herein does not reach 70 Bc as the passivated cement solutiondoes not set to form a hardened mass.

To form the passivated cement solution, water may be combined with acement in an amount such that the cement grains are diluted and notcapable of agglomerating to form a hardened mass. The amount of waterrequired to prepare a passivated cement solution may depend on the typeand origin of the cement used as cements may vary in the amount of waterrequired to hydrate the cement grains. A wide variety of cements may beused to prepare the passivated cement solution such as, withoutlimitation, Portland cements, pozzolana cements, gypsum cements, aluminacements, silica cements, and any combination thereof. The amount ofwater should be sufficient to dilute the water and cement mixture enoughso that the particles of the cement material generally do notagglomerate and bind to each other, i.e. they remain discrete. Anabundance of water should be used such that the particles of thecementitious material are not capable of agglomerating, for example thewater may be used in an amount of about 400% by weight of thecementitious material to about 5000% by weight of the cementitiousmaterial or more. Alternatively, the water may be used in an amount ofabout 400% by weight of the cementitious material to about 1000% byweight of the cementitious material, about 1000% by weight of thecementitious material to about 2500% by weight of the cementitiousmaterial, about 2500% by weight of the cementitious material to about5000% by weight of the cementitious material, or any rangestherebetween. In the Examples below, the water is present in about 1000%by weight of water.

The water may be provided in an amount such that particles of the cementmaterial are able to not agglomerate. Agglomerated particles may bebroken by shearing and suspension aids may be used to keep the particlesfrom settling. After the water has been added to the cement thepassivation process will commence. The mixture may need to react for aperiod, for example, of about 1 hours to about 24 hours. Alternatively,the mixture may be reacted for about 1 hour to about 2 hours, about 2hours to about 5 hours, or about 5 hours to about 24 hours, or anyranges therebetween for example. The passivation process may be carriedout at any temperature, for example temperatures ranging from about 5°C. to about 80° C. Alternatively, from about 5° C. to about 20° C.,about 20° C. to about 60° C., about 60° C. to about 80° C., or anyranges therebetween. During the reaction phase, the mixture may to bestirred either continuously or intermittently or may be kept in aquiescent state. Any type of stirring or agitation may be used includingmagnetic stirrers and overhead stirrers for example. Additionally, asuspension agent, may be used to aid in suspending the cement particles.Use of the suspending agent may be in addition to or in substitution ofagitation. Examples of suitable suspending aids may includeviscosifiers, such as those described above which include swellableclays such as bentonite or biopolymers such as cellulose derivatives(e.g., hydroxyethyl cellulose, carboxymethyl cellulose, carboxymethylhydroxyethyl cellulose). The passivation process may include manyindividual cementitious reactions, the cement hydration products ofwhich may depend on the particular cement selected. With Portlandcement, some cement hydration products may include a mixture ofpartially and fully reacted cement grains, C—S—H (calcium silicatehydrate) micro and/or nanoparticles, and a solution pH of greater than7.

Once the passivated cement solution is prepared, a suitable raw pozzolanis added to the passivated cement solution. Raw pozzolans may includeany described above, including but not limited to, fly ash, volcanicash, tuft, pumicites, clays such as metakaolin, silica fume, slag, limeash, perlite, and glass such as silicate glass, soda-lime glass,soda-silica glass, borosilicate glass, and aluminosilicate glass. Theraw pozzolan may be added in any suitable amount including from about100% by weight of cement in the passivated cement solution to about2000% by weight of cement in the passivated cement solution.Alternatively, from about 100% by weight of cement in the passivatedcement solution to about 500% by weight of cement in the passivatedcement solution, from about 500% by weight of cement in the passivatedcement solution to about 1000% by weight of cement in the passivatedcement solution, from about 1000% by weight of cement in the passivatedcement solution to about 2000% by weight of cement in the passivatedcement solution, or any ranges therebetween. Once the raw pozzolans areadded to the passivated cement solution, the cement hydration productsin the passivated cement solution begin to react with the raw pozzolanto produce activated pozzolan.

The mixture may need to react for a period, for example, of about 1hours to about 24 hours. Alternatively, the mixture may be reacted forabout 1 hour to about 2 hours, about 2 hours to about 5 hours, about 5hours to about 12 hours, about 12 hours to about 24 hours or any rangestherebetween for example. The reaction may be carried out at anytemperature, for example temperatures ranging from about 5° C. to about80° C. Alternatively, from about 5° C. to about 20° C., about 20° C. toabout 60° C., about 60° C. to about 80° C., or any ranges therebetween.During the activation reaction phase, the mixture may to be stirredeither continuously or intermittently or may be kept in a quiescentstate. the mixture may to be stirred either continuously orintermittently or may be kept in a quiescent state. Additionally, asuspension agent, may be used to aid in suspending the raw pozzolan. Useof the suspending agent may be in addition to or in substitution ofagitation. Examples of suitable suspending aids may includeviscosifiers, such as those described above which include swellableclays such as bentonite or biopolymers such as cellulose derivatives(e.g., hydroxyethyl cellulose, carboxymethyl cellulose, carboxymethylhydroxyethyl cellulose).

Activated pozzolan may undergo surface reactions in the passivatedcement solution. Without being limited by theory, it is believed thepassivated cement solution deposits calcium silicate hydrates andcarbonates on the surface of the raw pozzolan which increases thereactivity of the raw pozzolan. The passivated cement solution may etchand dissolve the surface of the raw pozzolan to create soluble speciesand induce formation of nucleation sites on the raw pozzolan.

The product mixture from the activation reaction may include unreactedpassivated cement solution, unreacted pozzolan, and activated pozzolan.The product mixture may be dried, granulated, and sieved, if desired.The dry activated pozzolan product produced from the drying step may beutilized in cement bulk blending as a dry additive. Cement dry blendsmay be prepared in bulk blending facilities where a cement, such asPortland cement, may be mixed with additives such as supplementarycementitious materials, chemical additives, inert additives, and theactivated pozzolan. The cement dry blend may be transported to awellbore location where the cement dry blend is mixed with water to forma cement slurry which is introduced into a wellbore.

Cement slurries described herein may generally include a hydrauliccement and water. A variety of hydraulic cements may be utilized inaccordance with the present disclosure, including, but not limited to,those comprising calcium, aluminum, silicon, oxygen, iron, and/orsulfur, which set and harden by reaction with water. Suitable hydrauliccements may include, but are not limited to, Portland cements, pozzolanacements, gypsum cements, alumina cements, silica cements, and anycombination thereof. In certain examples, the hydraulic cement mayinclude a Portland cement. In some examples, the Portland cements mayinclude Portland cements that are classified as Classes A, C, H, and Gcements according to American Petroleum Institute, API Specification forMaterials and Testing for Well Cements, API Specification 10, Fifth Ed.,Jul. 1, 1990. In addition, hydraulic cements may include cementsclassified by American Society for Testing and Materials (ASTM) in C150(Standard Specification for Portland Cement), C595 (StandardSpecification for Blended Hydraulic Cement) or C1157 (PerformanceSpecification for Hydraulic Cements) such as those cements classified asASTM Type I, II, or III. The hydraulic cement may be included in thecement slurry in any amount suitable for a particular composition.Without limitation, the hydraulic cement may be included in the cementslurries in an amount in the range of from about 10% to about 80% byweight of dry blend in the cement slurry. For example, the hydrauliccement may be present in an amount ranging between any of and/orincluding any of about 10%, about 15%, about 20%, about 25%, about 30%,about 35%, about 40%, about 45%, about 50%, about 55%, about 60%, about65%, about 70%, about 75%, or about 80% by weight of the cementslurries.

The water may be from any source provided that it does not contain anexcess of compounds that may undesirably affect other components in thecement slurries. For example, a cement slurry may include fresh water orsaltwater. Saltwater generally may include one or more dissolved saltstherein and may be saturated or unsaturated as desired for a particularapplication. Seawater or brines may be suitable for use in someexamples. Further, the water may be present in an amount sufficient toform a pumpable slurry. In certain examples, the water may be present inthe cement slurry in an amount in the range of from about 33% to about200% by weight of the cementitious materials. For example, the watercement may be present in an amount ranging between any of and/orincluding any of about 33%, about 50%, about 75%, about 100%, about125%, about 150%, about 175%, or about 200% by weight of thecementitious materials. The cementitious materials referenced mayinclude all components which contribute to the compressive strength ofthe cement slurry such as the hydraulic cement and supplementarycementitious materials, for example.

As mentioned above, the cement slurry may include supplementarycementitious materials. The supplementary cementitious material may beany material that contributes to the desired properties of the cementslurry. Some supplementary cementitious materials may include, withoutlimitation, fly ash, blast furnace slag, silica fume, pozzolans, kilndust, and clays, for example.

The cement slurry may include kiln dust as a supplementary cementitiousmaterial. “Kiln dust,” as that term is used herein, refers to a solidmaterial generated as a by-product of the heating of certain materialsin kilns. The term “kiln dust” as used herein is intended to includekiln dust made as described herein and equivalent forms of kiln dust.Depending on its source, kiln dust may exhibit cementitious propertiesin that it can set and harden in the presence of water. Examples ofsuitable kiln dusts include cement kiln dust, lime kiln dust, andcombinations thereof. Cement kiln dust may be generated as a by-productof cement production that is removed from the gas stream and collected,for example, in a dust collector. Usually, large quantities of cementkiln dust are collected in the production of cement that are commonlydisposed of as waste. The chemical analysis of the cement kiln dust fromvarious cement manufactures varies depending on a number of factors,including the particular kiln feed, the efficiencies of the cementproduction operation, and the associated dust collection systems. Cementkiln dust generally may include a variety of oxides, such as SiO₂,Al₂O₃, Fe₂O₃, CaO, MgO, SO₃, Na₂O, and K₂O. The chemical analysis oflime kiln dust from various lime manufacturers varies depending onseveral factors, including the particular limestone or dolomiticlimestone feed, the type of kiln, the mode of operation of the kiln, theefficiencies of the lime production operation, and the associated dustcollection systems. Lime kiln dust generally may include varying amountsof free lime and free magnesium, lime stone, and/or dolomitic limestoneand a variety of oxides, such as SiO₂, Al₂O₃, Fe₂O₃, CaO, MgO, SO₃,Na₂O, and K₂O, and other components, such as chlorides. A cement kilndust may be added to the cement slurry prior to, concurrently with, orafter activation. Cement kiln dust may include a partially calcined kilnfeed which is removed from the gas stream and collected in a dustcollector during the manufacture of cement. The chemical analysis of CKDfrom various cement manufactures varies depending on a number offactors, including the particular kiln feed, the efficiencies of thecement production operation, and the associated dust collection systems.CKD generally may comprise a variety of oxides, such as SiO₂, Al₂O₃,Fe₂O₃, CaO, MgO, SO₃, Na₂O, and K₂O. The CKD and/or lime kiln dust maybe included in examples of the cement slurry in an amount suitable for aparticular application.

In some examples, the cement slurry may further include one or more ofslag, natural glass, shale, amorphous silica, or metakaolin as asupplementary cementitious material. Slag is generally a granulated,blast furnace by-product from the production of cast iron including theoxidized impurities found in iron ore. The cement may further includeshale. A variety of shales may be suitable, including those includingsilicon, aluminum, calcium, and/or magnesium. Examples of suitableshales include vitrified shale and/or calcined shale. In some examples,the cement slurry may further include amorphous silica as asupplementary cementitious material. Amorphous silica is a powder thatmay be included in embodiments to increase cement compressive strength.Amorphous silica is generally a byproduct of a ferrosilicon productionprocess, wherein the amorphous silica may be formed by oxidation andcondensation of gaseous silicon suboxide, SiO, which is formed as anintermediate during the process

In some examples, the cement slurry may further include a variety of flyashes as a supplementary cementitious material which may include fly ashclassified as Class C, Class F, or Class N fly ash according to AmericanPetroleum Institute, API Specification for Materials and Testing forWell Cements, API Specification 10, Fifth Ed., Jul. 1, 1990. In someexamples, the cement slurry may further include zeolites assupplementary cementitious materials. Zeolites are generally porousalumino-silicate minerals that may be either natural or synthetic.Synthetic zeolites are based on the same type of structural cell asnatural zeolites and may comprise aluminosilicate hydrates. As usedherein, the term “zeolite” refers to all natural and synthetic forms ofzeolite.

Where used, one or more of the aforementioned supplementary cementitiousmaterials may be present in the cement slurry. For example, withoutlimitation, one or more supplementary cementitious materials may bepresent in an amount of about 0.1% to about 80% by weight of the cementslurry. For example, the supplementary cementitious materials may bepresent in an amount ranging between any of and/or including any ofabout 0.1%, about 10%, about 20%, about 30%, about 40%, about 50%, about60%, about 70%, or about 80% by weight of the cement.

In some examples, the cement slurry may further include hydrated lime.As used herein, the term “hydrated lime” will be understood to meancalcium hydroxide. In some embodiments, the hydrated lime may beprovided as quicklime (calcium oxide) which hydrates when mixed withwater to form the hydrated lime. The hydrated lime may be included inexamples of the cement slurry, for example, to form a hydrauliccomposition with the supplementary cementitious components. For example,the hydrated lime may be included in a supplementary cementitiousmaterial-to-hydrated-lime weight ratio of about 10:1 to about 1:1 or 3:1to about 5:1. Where present, the hydrated lime may be included in theset cement slurry in an amount in the range of from about 10% to about100% by weight of the cement slurry, for example. In some examples, thehydrated lime may be present in an amount ranging between any of and/orincluding any of about 10%, about 20%, about 40%, about 60%, about 80%,or about 100% by weight of the cement slurry. In some examples, thecementitious components present in the cement slurry may consistessentially of one or more supplementary cementitious materials and thehydrated lime. For example, the cementitious components may primarilycomprise the supplementary cementitious materials and the hydrated limewithout any additional components (e.g., Portland cement, fly ash, slagcement) that hydraulically set in the presence of water.

Lime may be present in the cement slurry in several; forms, including ascalcium oxide and or calcium hydroxide or as a reaction product such aswhen Portland cement reacts with water. Alternatively, lime may beincluded in the cement slurry by amount of silica in the cement slurry.A cement slurry may be designed to have a target lime to silica weightratio. The target lime to silica ratio may be a molar ratio, molalratio, or any other equivalent way of expressing a relative amount ofsilica to lime. Any suitable target time to silica weight ratio may beselected including from about 10/90 lime to silica by weight to about40/60 lime to silica by weight. Alternatively, about 10/90 lime tosilica by weight to about 20/80 lime to silica by weight, about 20/80lime to silica by weight to about 30/70 lime to silica by weight, orabout 30/70 lime to silica by weight to about 40/63 lime to silica byweight.

Other additives suitable for use in subterranean cementing operationsalso may be included in embodiments of the cement slurry. Examples ofsuch additives include, but are not limited to: weighting agents,lightweight additives, gas-generating additives,mechanical-property-enhancing additives, lost-circulation materials,filtration-control additives, fluid-loss-control additives, defoamingagents, foaming agents, thixotropic additives, and combinations thereof.In embodiments, one or more of these additives may be added to thecement slurry after storing but prior to the placement of a cementslurry into a subterranean formation. In some examples, the cementslurry may further include a dispersant. Examples of suitabledispersants include, without limitation, sulfonated-formaldehyde-baseddispersants (e.g., sulfonated acetone formaldehyde condensate) orpolycarboxylated ether dispersants. In some examples, the dispersant maybe included in the cement slurry in an amount in the range of from about0.01% to about 5% by weight of the cementitious materials. In specificexamples, the dispersant may be present in an amount ranging between anyof and/or including any of about 0.01%, about 0.1%, about 0.5%, about1%, about 2%, about 3%, about 4%, or about 5% by weight of thecementitious materials.

In some examples, the cement slurry may further include a set retarder.A broad variety of set retarders may be suitable for use in the cementslurries. For example, the set retarder may comprise phosphonic acids,such as ethylenediamine tetra(methylene phosphonic acid),diethylenetriamine penta(methylene phosphonic acid), etc.;lignosulfonates, such as sodium lignosulfonate, calcium lignosulfonate,etc.; salts such as stannous sulfate, lead acetate, monobasic calciumphosphate, organic acids, such as citric acid, tartaric acid, etc.;cellulose derivatives such as hydroxyl ethyl cellulose (HEC) andcarboxymethyl hydroxyethyl cellulose (CMHEC); synthetic co- orter-polymers comprising sulfonate and carboxylic acid groups such assulfonate-functionalized acrylamide-acrylic acid co-polymers; boratecompounds such as alkali borates, sodium metaborate, sodium tetraborate,potassium pentaborate; derivatives thereof, or mixtures thereof.Examples of suitable set retarders include, among others, phosphonicacid derivatives. Generally, the set retarder may be present in thecement slurry in an amount sufficient to delay the setting for a desiredtime. In some examples, the set retarder may be present in the cementslurry in an amount in the range of from about 0.01% to about 10% byweight of the cementitious materials. In specific examples, the setretarder may be present in an amount ranging between any of and/orincluding any of about 0.01%, about 0.1%, about 1%, about 2%, about 4%,about 6%, about 8%, or about 10% by weight of the cementitiousmaterials.

In some examples, the cement slurry may further include an accelerator.A broad variety of accelerators may be suitable for use in the cementslurries. For example, the accelerator may include, but are not limitedto, aluminum sulfate, alums, calcium chloride, calcium nitrate, calciumnitrite, calcium formate, calcium sulphoaluminate, calcium sulfate,gypsum-hemihydrate, sodium aluminate, sodium carbonate, sodium chloride,sodium silicate, sodium sulfate, ferric chloride, or a combinationthereof. In some examples, the accelerators may be present in the cementslurry in an amount in the range of from about 0.01% to about 10% byweight of the cementitious materials. In specific examples, theaccelerators may be present in an amount ranging between any of and/orincluding any of about 0.01%, about 0.1%, about 1%, about 2%, about 4%,about 6%, about 8%, or about 10% by weight of the cementitiousmaterials.

Cement slurries generally should have a density suitable for aparticular application. By way of example, the cement slurry may have adensity in the range of from about 8 pounds per gallon (“ppg”) (959kg/m³) to about 20 ppg (2397 kg/m³), or about 8 ppg to about 12 ppg(1437. kg/m³), or about 12 ppg to about 16 ppg (1917.22 kg/m³), or about16 ppg to about 20 ppg, or any ranges therebetween. Examples of thecement slurry may be foamed or unfoamed or may comprise other means toreduce their densities, such as hollow microspheres, low-density elasticbeads, or other density-reducing additives known in the art.

FIG. 1 illustrates an example system 5 for preparation of a cementslurry including and delivery of the cement slurry to a wellbore. Thecement slurry may be any cement slurry disclosed herein including thosecomprising an activated pozzolan. As shown, the cement slurry may bemixed in mixing equipment 10, such as a jet mixer, re-circulating mixer,or a batch mixer, for example, and then pumped via pumping equipment 15to the wellbore. In some examples, the mixing equipment 10 and thepumping equipment 15 may be disposed on one or more cement trucks aswill be apparent to those of ordinary skill in the art. In someexamples, a jet mixer may be used, for example, to continuously mix adry blend including the cement slurry, for example, with the water as itis being pumped to the wellbore.

An example technique for placing a cement slurry into a subterraneanformation will now be described with reference to FIGS. 2 and 3 . FIG. 2illustrates example surface equipment 20 that may be used in placementof a cement slurry. The cement slurry may be any cement slurry disclosedherein. A cement slurry recipe be developed, for example, using thecement fluid loss models described herein, and a cement slurry may beprepared based on the cement slurry recipe. It should be noted thatwhile FIG. 2 generally depicts a land-based operation, those skilled inthe art will readily recognize that the principles described herein areequally applicable to subsea operations that employ floating orsea-based platforms and rigs, without departing from the scope of thedisclosure. As illustrated by FIG. 2 , the surface equipment 20 mayinclude a cementing unit 25, which may include one or more cementtrucks. The cementing unit 25 may include mixing equipment 10 andpumping equipment 15 (e.g., FIG. 1 ) as will be apparent to those ofordinary skill in the art. The cementing unit 25 may pump a cementslurry 30 through a feed pipe 35 and to a cementing head 36 whichconveys the cement slurry 30 downhole.

Turning now to FIG. 3 , the cement slurry 30, may be placed into asubterranean formation 45. As illustrated, a wellbore 50 may be drilledinto one or more subterranean formations 45. While the wellbore 50 isshown extending generally vertically into the one or more subterraneanformation 45, the principles described herein are also applicable towellbores that extend at an angle through the one or more subterraneanformations 45, such as horizontal and slanted wellbores. As illustrated,the wellbore 50 includes walls 55. In the illustrated example, a surfacecasing 60 has been inserted into the wellbore 50. The surface casing 60may be cemented to the walls 55 of the wellbore 50 by cement sheath 65.In the illustrated example, one or more additional conduits (e.g.,intermediate casing, production casing, liners, etc.), shown here ascasing 70 may also be disposed in the wellbore 50. As illustrated, thereis a wellbore annulus 75 formed between the casing 70 and the walls 55of the wellbore 50 and/or the surface casing 60. One or morecentralizers 80 may be attached to the casing 70, for example, tocentralize the casing 70 in the wellbore 50 prior to and during thecementing operation.

With continued reference to FIG. 3 , the cement slurry 30 may be pumpeddown the interior of the casing 70. The cement slurry 30 may be allowedto flow down the interior of the casing 70 through the casing shoe 85 atthe bottom of the casing 70 and up around the casing 70 into thewellbore annulus 75. The cement slurry 30 may be allowed to set in thewellbore annulus 75, for example, to form a cement sheath that supportsand positions the casing 70 in the wellbore 50. While not illustrated,other techniques may also be utilized for introduction of the cementslurry 30. By way of example, reverse circulation techniques may be usedthat include introducing the cement slurry 30 into the subterraneanformation 45 by way of the wellbore annulus 75 instead of through thecasing 70.

As it is introduced, the cement slurry 30 may displace other fluids 90,such as drilling fluids and/or spacer fluids that may be present in theinterior of the casing 70 and/or the wellbore annulus 75. At least aportion of the displaced fluids 90 may exit the wellbore annulus 75 viaa flow line 95 and be deposited, for example, in one or more retentionpits 100 (e.g., a mud pit), as shown on FIG. 2 . Referring again to FIG.3 , a bottom plug 105 may be introduced into the wellbore 50 ahead ofthe cement slurry 30, for example, to separate the cement slurry 30 fromthe other fluids 90 that may be inside the casing 70 prior to cementing.After the bottom plug 105 reaches the landing collar 110, a diaphragm orother suitable device should rupture to allow the cement slurry 30through the bottom plug 105. In FIG. 3 , the bottom plug 105 is shown onthe landing collar 110. In the illustrated example, a top plug 115 maybe introduced into the wellbore 50 behind the cement slurry 30. The topplug 115 may separate the cement slurry 30 from a displacement fluid 120and push the cement slurry 30 through the bottom plug 105.

The cement slurries disclosed herein may be used in a variety ofsubterranean applications, including primary and remedial cementing. Thecement slurries may be introduced into a subterranean formation andallowed to set. In primary cementing applications, for example, thecement slurries may be introduced into the annular space between aconduit located in a wellbore and the walls of the wellbore (and/or alarger conduit in the wellbore), wherein the wellbore penetrates thesubterranean formation. The cement slurry may be allowed to set in theannular space to form an annular sheath of hardened cement. The cementslurry may form a barrier that prevents the migration of fluids in thewellbore. The cement slurry may also, for example, support the conduitin the wellbore. In remedial cementing applications, the cement slurrymay be used, for example, in squeeze cementing operations or in theplacement of cement plugs. By way of example, the cement slurry may beplaced in a wellbore to plug an opening (e.g., a void or crack) in theformation, in a gravel pack, in the conduit, in the cement sheath,and/or between the cement sheath and the conduit (e.g., a microannulus), and/or in a reverse cementing application.

The following statements may describe certain embodiments of thedisclosure but should be read to be limiting to any particularembodiment.

-   -   Statement 1. A method of cementing comprising: preparing a        cement slurry by mixing at least water and a cement dry blend,        wherein the cement dry blend comprises a cement and an activated        pozzolan; and introducing the cement slurry into a wellbore        penetrating a subterranean formation; and allowing the cement        slurry to set to form a hardened mass.    -   Statement 2. The method of statement 1 wherein the cement        comprises at least one selected from the group consisting of a        Portland cement, a pozzolana cement, a gypsum cement, an alumina        cement, a silica cements, and combinations thereof.    -   Statement 3. The method of any of statements 1-2 wherein the        activated pozzolan comprises cement hydration products deposited        on a pozzolan, wherein the cementhydration products are reaction        products of water and at least one cement selected from the        group consisting of a Portland cement, a pozzolana cement, a        gypsum cement, an alumina cement, a silica cements, and        combinations thereof.    -   Statement 4. The method of any of statements 1-3 wherein the        pozzolan is selected from the group consisting of fly ash,        volcanic ash, tuft, pumicites, metakaolin, silica fume, slag,        lime ash, perlite, silicate glass, soda-lime glass, soda-silica        glass, borosilicate glass, aluminosilicate glass, aplite, clays,        calcined clays, and combinations thereof.    -   Statement 5. The method of any of statements 1-4 wherein the        cement hydration products comprise microcrystalline and/or        nanocrystalline calcium silica hydrate.    -   Statement 6. The method of any of statements 1-5 wherein cement        hydration products comprise carbonate.    -   Statement 7. A method of producing an activated pozzolan        comprising: mixing at least a raw pozzolan and a passivated        cement solution, wherein the passivated cement solution        comprising cement hydration products; reacting at least a        portion of the raw pozzolan with the cement hydration products        in the passivated cement solution to produce an activated        pozzolan; drying the activated pozzolan; and blending the        activated pozzolan with a cement.    -   Statement 8. The method of statement 7 further comprising:        mixing at least a second cement and water to form a        pre-passivated cement solution, wherein the water is present in        an amount such that the pre-passivated solution does not set to        form a hardened mass; and reacting at least a portion of the        second cement with the water to produce the passivated cement        solution comprising the cement hydration products.    -   Statement 9. The method of any of statements 7-8 wherein the        second cement comprises at least one selected from the group        consisting of a Portland cement, a pozzolana cement, a gypsum        cement, an alumina cement, a silica cements, and combinations        thereof.    -   Statement 10. The method of any of statements 7-9 wherein the        water is present in an amount of about 400% by weight of the        second cement to about 5000% by weight of the second cement.    -   Statement 11. The method of any of statements 7-10 wherein the        reacting the second cement with the water to produce the        passivated cement solution is carried out at a temperature of        about 5° C. to about 80° C.    -   Statement 12. The method of any of statements 7-11 wherein the        reacting the second cement with the water to produce the        passivated cement solution is carried out for about 1 hour to        about 24 hours.    -   Statement 13. The method of any of statements 7-12 wherein the        cement hydration products comprise microcrystalline and/or        nanocrystalline calcium silica hydrate.    -   Statement 14. The method of any of statements 7-13 wherein        cement hydration products comprise a carbonate.    -   Statement 15. The method of any of statements 7-14 wherein the        cement hydration products comprise at least reacted cement        grains.    -   Statement 16. The method of any of statements 7-15 wherein the        raw pozzolan is selected from the group consisting of fly ash,        volcanic ash, tuft, pumicites, metakaolin, silica fume, slag,        lime ash, perlite, silicate glass, soda-lime glass, soda-silica        glass, borosilicate glass, aluminosilicate glass, aplite, clays,        calcined clays and combinations thereof.    -   Statement 17. The method of any of statements 7-16 wherein the        cement comprises at least one selected from the group consisting        of a Portland cement, a pozzolana cement, a gypsum cement, an        alumina cement, a silica cements, and combinations thereof.    -   Statement 18. The method of any of statements 7-17 wherein the        blending the activated pozzolan with a cement further comprises        blending with at least of a supplementary cementitious material,        a chemical additive, an inert additive.    -   Statement 19. A composition comprising: a cement; and an        activated pozzolan, wherein the cement and the activated        pozzolan are a dry mixed powder.    -   Statement 20. The composition of statement 19 wherein the        activated pozzolan comprises cement hydration products disposed        on a surface of a pozzolan, the cement hydration products        comprising at least one hydration product selected from the        group consisting of microcrystalline calcium silica hydrate,        nanocrystalline calcium silica hydrate, a carbonate, fully        reacted cement grains, partially reacted cement grains, and        combinations thereof.

EXAMPLES

To facilitate a better understanding of the present disclosure, thefollowing examples of certain aspects of some embodiments are given. Inno way should the following examples be read to limit, or define, theentire scope of the disclosure.

Example 1

Cements were prepared using an activated pozzolan as described herein.Recycled glass was treated with the methods described to produce anactivated pozzolan. The recycled glass used in this example was obtainedas crushed particles from Strategic Materials, Inc with a measuredparticle size distribution D₅₀ of 80 microns, a measured specificgravity of 2.54, and a water requirement of 59.

To enhance pozzolanic activity of the recycled glass particles, theparticles were treated by the following method. First a passivatedcement slurry was formed by mixing 450.0 g water with 45.0 g Class Gcement in a 1.0 liter bottle. The bottle containing the cement slurrywas placed in a 160° F. oven to react the cement particles. After twohours the bottle was removed from the oven and 400.0 g of the recycledglass particles were added to the mixture and mixed. The bottle wasplaced back in the oven at 160° F. for two additional hours. The bottlewas then removed from the oven and the contents poured into a 10″ by 10″cake pan and placed in an oven at 190° F. for 24 hours to obtain a drypowder activated pozzolan.

Two cement slurries were prepared according to Table 1 and Table 2. Thefirst cement slurry utilized the raw recycled glass powder as apozzolanic additive and the second cement slurry utilized the activatedpozzolan.

TABLE 1 Material Weight (g) SG Vol. (ml) PPG Texas Lehigh Type I 64.83.2 20.1 13.9 Raw Recycled Glass 259.2 2.6 101.6 Water 178.2 1 178.6

TABLE 2 Material Weight (g) SG Vol. (ml) PPG Texas Lehigh Type I 64.83.2 20.1 13.9 Activated Pozzolan 259.2 2.6 101.6 Water 178.2 1 178.6

The slurries from Table 1 and Table 2 equate to 80% by weight of dryblend recycled glass and 20% by weight of dry blend cement, and 55%water by weight of blend (bwob). Both slurries were mixed according toAPI specifications and placed in separate ultrasonic cement analyzers(UCAs) both at 140° F. and 3000 psi. After about 96 hours the set cementsamples were removed from the UCA and crushed using a Tinius-Olsenload-frame. The crush value was then used to correct the UCA results,shown in FIG. 4 . Table 3 shows the tabulated valued from crushcorrected UCA.

TABLE 3 Time at Crush Corrected C.S. (psi) Material 50 psi (hr) 24 hr 48hr 72 hr 96 hr Raw Recycled Glass Cement 5.18 196 465 556 581 ActivatedPozzolan 4.37 252 664 910 957 % Improvement 15.6 28.6 42.8 63.7 64.7

It can be observed from FIG. 4 and Table 3 that the slurry containingthe activated pozzolan derived from raw recycled glass has a greatercompressive strength than the slurry containing the raw recycled glass.

Example 2

Cements were prepared using an activated pozzolan as described herein.Perlite powder was treated with the methods described to produce anactivated pozzolan. The perlite used in this example was from Imerys N.A., Inc, Inc and had a measured particle size distribution D50 of 16microns, a measured specific gravity of 2.43, and a water requirement of52.

To enhance pozzolanic activity of the perlite powder, the particles weretreated by the following method. First a passivated cement slurry wasformed by mixing 450.0 g water with 45.0 g Class G cement in a 1.0 literbottle. The bottle containing cement slurry was placed in a 160° F. ovento react the cement particles. After two hours the bottle was removedfrom the oven and 400.0 g of the perlite powder was added to the mixtureand mixed. The bottle was placed back in the over at 160° F. for twoadditional hours. The bottle was then removed from the oven and thecontents poured into a 10″ by 10″ cake pan and placed in an oven at 190°F. for 24 hours to obtain a dry powdered activated pozzolan.

Two cement slurries were prepared according to Table 4 and Table 5. Thefirst cement slurry utilized the raw perlite powder as a pozzolanicadditive and the second cement slurry utilized the activated pozzolanproduced from raw perlite.

TABLE 4 Material Weight (g) SG Vol. (ml) PPG Texas Lehigh Type I 64.03.2 19.8 13.7 Raw Perlite 256.0 2.5 104.5 Water 176 1 176.4

TABLE 5 Material Weight (g) SG Vol. (ml) PPG Texas Lehigh Type I 64.03.2 19.8 13.7 Activated Pozzolan 256.0 2.5 104.5 Water 176 1 176.4

The slurries from Table 4 and Table 5 equate to 80% by weight of dryblend perlite, 20% by weight of dry blend cement, and 55% by weight ofblend (bwob). Both slurries were mixed according to API specificationsand placed in separate UCAs both at 140° F. and 3000 psi. After about 96hours the set cement samples were removed from the UCA and crushed usinga Tinius-Olsen load-frame. The crush value was then used to correct theUCA results, shown in FIG. 5 . Table 3 shows the tabulated valued fromcrush corrected UCA.

TABLE 6 Time at Crush Corrected C.S. (psi) Material 50 psi (hr) 24 hr 48hr 72 hr 96 hr Raw Perlite Cement 2.50 882 932 950 954 ActivatedPozzolan 2.66 1456 1696 1800 1875 Cement % Improvement −6.4 65.1 82.089.5 96.5

It can be observed from FIG. 5 and Table 6 that the slurry containingthe activated pozzolan derived from raw perlite has a greatercompressive strength than the slurry containing the raw perlite.

For the sake of brevity, only certain ranges are explicitly disclosedherein. However, ranges from any lower limit may be combined with anyupper limit to recite a range not explicitly recited, as well as rangesfrom any lower limit may be combined with any other lower limit torecite a range not explicitly recited, in the same way, ranges from anyupper limit may be combined with any other upper limit to recite a rangenot explicitly recited. Additionally, whenever a numerical range with alower limit and an upper limit is disclosed, any number and any includedrange falling within the range are specifically disclosed. Inparticular, every range of values (of the form, “from about a to aboutb,” or, equivalently, “from approximately a to b,” or, equivalently,“from approximately a-b”) disclosed herein is to be understood to setforth every number and range encompassed within the broader range ofvalues even if not explicitly recited. Thus, every point or individualvalue may serve as its own lower or upper limit combined with any otherpoint or individual value or any other lower or upper limit, to recite arange not explicitly recited.

Therefore, the present disclosure is well adapted to attain the ends andadvantages mentioned as well as those that are inherent therein. Theparticular examples disclosed above are illustrative only, as thepresent disclosure may be modified and practiced in different butequivalent manners apparent to those skilled in the art having thebenefit of the teachings herein. Although individual examples arediscussed, the disclosure covers all combinations of all those examples.Furthermore, no limitations are intended to the details of constructionor design herein shown, other than as described in the claims below.Also, the terms in the claims have their plain, ordinary meaning unlessotherwise explicitly and clearly defined by the patentee. It istherefore evident that the particular illustrative examples disclosedabove may be altered or modified and all such variations are consideredwithin the scope and spirit of the present disclosure. If there is anyconflict in the usages of a word or term in this specification and oneor more patent(s) or other documents that may be incorporated herein byreference, the definitions that are consistent with this specificationshould be adopted.

What is claimed is:
 1. A method comprising: preparing a cement slurry bymixing at least water and a cement dry blend, wherein the cement dryblend comprises a cement and an activated pozzolan, wherein theactivated pozzolan comprises particles of pozzolan having cementhydration products deposited on a surface of the pozzolan particles andwherein the activated pozzolan is produced by a process comprising:mixing at least a raw pozzolan and a passivated cement solution, whereinthe passivated cement solution comprises cement hydration products;reacting at least a portion of the raw pozzolan with the cementhydration products in the passivated cement solution to produce theactivated pozzolan; and drying the activated pozzolan; and introducingthe cement slurry into a wellbore penetrating a subterranean formation;and allowing the cement slurry to set to form a hardened mass.
 2. Themethod of claim 1, wherein the cement comprises at least one selectedfrom the group consisting of a Portland cement, a pozzolana cement, agypsum cement, an alumina cement, a silica cements, and combinationsthereof.
 3. The method of claim 1, wherein the cement hydration productsare reaction products of water and at least one cement selected from thegroup consisting of a Portland cement, a pozzolana cement, a gypsumcement, an alumina cement, a silica cements, and combinations thereof.4. The method of claim 3, wherein the pozzolan is selected from thegroup consisting of fly ash, volcanic ash, tuft, pumicites, metakaolin,silica fume, slag, lime ash, perlite, silicate glass, soda-lime glass,soda-silica glass, borosilicate glass, aluminosilicate glass, aplite,clays, calcined clays, and combinations thereof.
 5. The method of claim3, wherein the cement hydration products comprise microcrystallineand/or nanocrystalline calcium silica hydrate.
 6. The method of claim 3,wherein the cement hydration products comprise carbonate.